1. Field of the Invention
The present invention relates to the fabrication of oxide films by the Atomic Layer Epitaxy (ALE) process, which is generally also known by the name of Atomic Layer Deposition (ALD). The invention relates in particular to a process for the fabrication of thin oxide films by the ALD method by using an oxygen-containing organometallic compound as the oxygen source material.
2. Description of the Related Art
The continual decrease in size of microelectronics components is leading to a situation in which SiO2 can no longer be used as the gate oxide in metal oxide semiconductor field effect transistors (MOSFET), since, in order to achieve the required capacitances, the SiO2 layer should be made so thin that the tunneling current would grow detrimentally high in terms of the functioning of the component. To avoid this problem, SiO2 has to be replaced with an insulator having a higher dielectric constant. In this case, the insulator may be in a thicker layer than SiO2.
Materials having sufficiently high dielectric constants are numerous, but there is the problem that the insulator must be prepared on top of silicon so that:
either a very thin or no SiO2 layer forms between the silicon and the insulator, which reduces the total capacitance according to the equation 1/C(tot)=1/C(insulator)+1/C(SiO2),
there are very few electrically active error states oh the interface between the silicon and the insulator, because the charge trapped in them has a negative effect on transistor properties such as threshold voltage, transconductance and stability, and
the thickness and uniformity of the insulator are precisely controlled.
Without a thin SiO2 layer on the interface between the silicon and the insulator it is difficult to produce a situation in which there would be very few electrically active error states on the interface. Thus it would be preferable that the structure of the interface would be an Si—SiO2 insulator wherein the thickness of the SiO2 layer would be one or two atom layers.
When an oxide film is deposited on silicon, the surface of the silicon tends to oxidize, forming a SiO2 layer which is too thick. The oxidation of silicon may be due to reactions with the oxygen source used in the process, reactions with the oxide insulator, or diffusion of oxygen. If the insulator selected is a metal oxide thermodynamically more stable than SiO2 (e.g. ZrO2, HfO2, Al2O3), oxidation can take place only under the effect of the oxygen source used in the deposition process. On the other hand, if the process is carried out at a sufficiently low temperature, the formation of an SiO2 layer may be hindered kinetically even if it were thermodynamically possible.
ALD (ALE, ALCVD) is a process, described, for example, in U.S. Pat. No. 4,058,430, for the growing of thin films. In the process, a thin film is deposited by means of alternating saturated surface reactions. These reactions are implemented by directing gaseous or vaporized source materials alternately into the reactor and by rinsing the reactor with an inert gas between the source material pulses (T. Suntola, Thin Solid Films 215 (1992) 84; Niinistö et al., Materials Science and Engineering B 41 (1996) 23). Since the film grows through saturated surface reactions, the growth is self-controlled, in which case controlling the film thickness by the number of reaction cycles is precise and simple. Films deposited by the ALD process are of uniform thickness over even large surface areas, and their conformality is excellent. Process technology and equipment are commercially supplied under the trade mark ALCVD™ by ASM Microchemistry Oy, Espoo, Finland.
The metal source materials used in the oxide processes in ALD are metal compounds of many types, in particular halides, alkoxides, β-diketonates, alkyls, or cyclopentadienyls (for example, M. Leskelä and M. Ritala, Journal de Physique IV 9 (1999) Pr8-837; Niinistö et al., Materials Science and Engineering B 41 (1996) 23). The most commonly used oxygen source materials are water, hydrogen peroxide and ozone. Alcohols, oxygen and nitrous oxide have also been used. Of these, oxygen reacts very poorly at the temperatures below 600° C. commonly used, but the other oxygen sources are highly reactive with most of the metal compounds listed above. On the other hand, these oxygen sources may also oxidize the silicon surface, thus producing the excessively thick intermediate layer of SiO2 previously described between the silicon and the oxide film.
The total reaction of a typical pair of source materials, a metal oxide and water, can be presented in the following formM(OR)a+a/2H2O→MOa/2+aROH,
where M stands for a metal, R is a hydrocarbon, and a is the valence of the metal M. Metal alkoxides may also break down thermally, producing the corresponding metal oxides. However, this is not desirable in the ALD process, because the most important characteristic—self-control of the deposition—is lost through thermal breakdown.
Drozd et al. in their article describe an ALD process in which an oxide made up of oxides of aluminum and chromium is produced by exposing a substrate consisting of a silicon crystal to alternate pulses of chromyl chromide (CrO2Cl2) and trimethyl aluminum (Al(CH3)3) (Drozd et al., Applied Surface Science 82/83 (1994) 587). However, relatively few metal oxochlorides are volatile at temperatures below 500° C., a fact which limits the usability of processes of this type in particular in applications of the semiconductor industry, where process temperatures above 500° C. are not desirable.
The object of the present invention is to eliminate the problems associated with the prior art and to provide an entirely novel solution to the problem of growing thin oxide films by ALD.